Effects of parathyroid hormone on puppies during development of Ca and vitamin D deficiency

The acute effects of parathyroid extract (PTE) were studied repeatedly in young dogs (prelabeled with 45Ca and [3H]tetracycline) during the development of calcium (Ca) and vitamin D deficiency. Blood Ca and radioactivity changes were monitored sequentially after subcutaneous PTE, injected seven times over 63 days. In control dogs, all sequential responses to acute PTE challenges were constant in both magnitude of increase and time at which maximum response occurred over the entire experiment. Under chronic Ca and D deficiency, plasma 25-hydroxyvitamin D in experimental dogs decreased continuously to very low levels at 63 days, whereas 1,25-dihydroxyvitamin D initially increased to a maximum at 32 days and thereafter decreased. In response to an acute challenge of PTE, dogs on the deficient diet for 3 and 10 days showed a greater response of blood Ca and 45Ca than the controls but subsequently showed a smaller response than controls after 49 and 63 days on the deficient diet. Compared with control dogs, the time of maximal response of blood Ca and 45Ca to PTE occurred much earlier in dogs that were on the deficient diet for 35-63 days. The blood [3H]tetracycline response (index of bone resorption) to exogenous PTE in the deficient dogs, however, was constant and similar to that of the control dogs during the entire period. The data suggest that the bone resorption response to PTE was normal in Ca- and D-deficient puppies with hypocalcemia.(ABSTRACT TRUNCATED AT 250 WORDS)